1. Field of the Invention
This invention relates to a method for altering the surface catalytic activity of synthetic crystalline zeolites (e.g. of the ZSM-5 type) which involves the sequential steps of synthesizing the crystalline zeolite from a synthesis reaction medium containing a source of bulky organic cations, contacting the "as synthesized", uncalcined crystalline zeolite product with hydrogen fluoride solution, calcining the hydrogen fluoride solution contacted zeolite, contacting the calcined zeolite with an ammonium exchange solution, and then calcining the ammonium exchange solution contacted zeolite.
2. Description of Prior Art
The use of zeolites as catalyst components is well known. Nevertheless, zeolites have been the subject of extensive investigations to improve their catalytic properties. Zeolites ZSM-5 and ZSM-11, for example, are fully described in U.S. Pat. Nos. 3,702,886 and 3,709,979, respectively, wherein it is disclosed that because of their ordered, porous structure, creating small interconnected cavities, they are selective toward certain molecules and provide catalytic capabilities for various chemical conversion reactions. U.S. Pat. Nos. 4,088,605 and 4,148,713 disclose a process combination wherein zeolites of the ZSM-5 type have their outer shells altered so as to be essentially aluminum-free, leading to a more selective catalyst. However, no known art discloses or suggests modifying the surface of synthetic zeolites, such as those of ZSM-5 type, by altering their surfaces and the alpha activity of the surfaces and thereby altering their catalytic properties by the present method.
It is noted that U.S. Pat. Nos. 3,354,078 and 3,644,220 relate to treating crystalline aluminosilicates with volatile metal halides. Neither of these latter patents, however, is concerned with altering surface activity of synthetic zeolites of ZSM-5 type having been prepared from reaction mixtures containing bulky ions as in the present method. In fact, the use of hydrogen fluoride with aluminosilicates has been avoided because of resulting lattice damage. Hydrogen fluoride in high concentrations, e.g., 5 N or greater, readily attacks both silica and alumina. Lower concentrations may also damage lattice structures if contact is maintained for too long a time. With some zeolitic materials, hydrogen fluoride treatment under controlled conditions has been used to alter pore size. U.S. Pat. Nos. 3,997,474 and 4,054,511 relate to altering effective pore size of natural ferrierite ore with very dilute hydrogen fluoride treatment. When the same treatment of erionite was conducted, a large loss in activity and crystallinity resulted.